The present invention relates to a method for the production of sodium percarbonate (abbreviated in the following to "PCS") with activated oxygen contents of at least 10% by weight and especially of more than 14.5% to 15.2% by weight, as well as to the PCS itself and the bleach and detergent compositions containing the new PCS product.
Sodium percarbonate is used as a bleaching component in powdered detergents, bleaches and cleaning agents. It is distinguished by a good water solubility as well as by a rapid release of hydrogen peroxide and is environmentally friendly, since its decomposition products do not contaminate the environment.
For sodium percarbonate, the empirical formula Na.sub.2 CO.sub.3.1.5 H.sub.2 O.sub.2 with a theoretical activated oxygen content of 15.28% by weight, is given in the literature. However, it should be taken into consideration here that sodium percarbonate, produced industrially from hydrogen peroxide and sodium carbonate, generally is not such a well-defined, homogeneous compound. Rather, on the one hand, it represents a mixture of compounds containing different amounts of water of hydration and having the formulas EQU Na.sub.2 CO.sub.3.1.5 H.sub.2 O.sub.2 EQU Na.sub.2 CO.sub.3.1.5 H.sub.2 O.sub.2.H.sub.2 O EQU Na.sub.2 CO.sub.3.2 H.sub.2 O.sub.2.H.sub.2 O EQU Na.sub.2 CO.sub.3.2 H.sub.2 O.sub.2 EQU Na.sub.2 CO.sub.3.x H.sub.2 O.sub.2
and, on the other, depending on the manufacturing process, additionally contains a certain proportion of non-oxidized sodium carbonate as well as further additives, such as sodium sulfate or-sodium chloride, which are the inevitable result of the manufacturing process. The properties of the product are determined decisively by the manufacturing conditions, as well as by the respective additives, not only in relation to the stability, but also, for example, with respect to the activated oxygen content, the solubility and the bulk density or particle size of the sodium percarbonate. For example, the attainable activated oxygen content in industrial grade sodium percarbonate is 13.4% to 14.5% by weight only in favorable cases; due to the additives inevitably resulting from the manufacturing process (sulfate, sodium chloride) as well as to the stabilizing measures, the attainable activated oxygen content frequently is much lower. The solubility of the sodium percarbonate, which is inherently good, is also frequently decreased, for example, by the presence of other salts, which inevitably result from the manufacturing process, such as sodium carbonate, sodium sulfate and sodium chloride. Moreover, the bulk density attainable or the particle size of the sodium percarbonate generally can be varied only slightly by the manufacturing methods of the state of the art and mostly is limited from the very start to a narrow range by the type of method or by the sodium carbonate used.
However, there has been an increasing desire for sodium percarbonates with a high activated oxygen content and different bulk densities or particle sizes to meet the different requirements of the detergent manufacturers, for example, for uses in light powder detergents with a low bulk density or in compact detergents with a high bulk density of the detergent, bleach and cleaning agent components. In particular, it is also necessary here to match the bulk densities of the individual components to one another, in order largely to preclude demixing, which would necessarily occur with components of different bulk densities.
Three technologies are known in the state of the art for the production of sodium percarbonate, namely crystallization methods, spraying methods and dry methods.
As a rule, sodium percarbonate is produced by the crystallization method. For this, a solution or suspension of sodium carbonate is reacted with hydrogen peroxide at 10.degree. to 20.degree. C. and crystallized in the presence of stabilizers, such as water glass, inorganic or organic phosphonic acids, etc. Because of the good solubility of the sodium percarbonate, it is, however necessary, for increasing the yield, to salt out the sodium percarbonate from the reaction mixture. For this purpose, according to the state of the art, preferably sodium chloride is added to the reaction mixture at a concentration of about 240 g/liter. It is, however, difficult to control the crystallization, so that, for the purpose of an advantageous shape of the crystal face, the addition of so-called crystallization improvers, such as polyphosphates or polyacrylates, is recommended. The crystallized sodium percarbonate is then centrifuged off and dried by conventional methods, for example, in a fluidized bed. However, the PCS, obtained by crystallization methods, is not optimum for many applications and its properties frequently suffer because it inevitably contains sodium chloride due to the manufacturing process.
For the spraying method of producing sodium percarbonate, it is not necessary to filter or centrifuge in order to remove the sodium percarbonate from the mother liquor. Rather, for this spraying method, an aqueous solution, or optionally also a suspension of low concentration of sodium carbonate and hydrogen peroxide, is dried in a spray drier. As a rule, however, spray dried products have a very low bulk density of only about 0.35 kg/liter and therefore cannot be used as such for the detergent formulations of today, which increasingly contain granular components having a higher bulk density. Moreover, much water must be removed when spraying solutions. This requires additional energy.
In modifications of the spraying method, solutions of sodium carbonate and hydrogen peroxide, for example, are sprayed continuously onto a bed of sodium percarbonate, previously put down and fluidized with hot air. The spraying and drying step can be carried out in one or two steps. In a further modification of the spraying method, solutions of sodium carbonate and hydrogen peroxide are fed through separate nozzles into a reaction chamber. A hot mixture of air and carbon dioxide is passed simultaneously through the reaction chamber. However, a fairly porous sodium percarbonate is obtained by this method, which does not meet the requirements for detergent compositions of the present standards with respect to bulk density and abrasion resistance.
According to the so-called dry methods, sodium percarbonate is produced by reacting hydrate water-free sodium carbonate with a concentrated solution of hydrogen peroxide of 50 to 80% by weight and evaporating the small amounts of water, which are released, already during the reaction. For this method, the reaction mixture is substantially dry during the whole of the reaction. The method can be carried out, for example, in mixers, fluidized bed reactors or also in tubular reactors with devices for feeding hydrogen peroxide. Aside from the long reaction times, this method has the disadvantage that there is no purification of the sodium carbonate produced in this manner, so that additional measures have to be taken to stabilize the product, for example, by adding special stabilizers already during the reaction. It is particularly disadvantageous that hydrogen peroxide must be used in a large excess, in order to obtain a PCS with an adequate content of activated oxygen. Moreover, this method is not very variable with respect to the granulate properties of the sodium percarbonate, for example, with respect to the bulk density and particle size, since the shape of the sodium percarbonate granulate corresponds essentially (that is, aside from slight roundings caused by the reaction) to the shape of the granulate of the sodium carbonate used. Therefore, especially when manufacturing sodium percarbonate granulates with a high bulk density for compact detergents, heavy sodium carbonate must be used, for which, however, only a little surface area is available for the reaction with hydrogen peroxide. The reaction is thus incomplete, so that only a lesser content of activated oxygen, as well as only inhomogeneous products with a higher proportion of sodium carbonate, which is distributed inhomogeneously, are obtained. Moreover, the alkalinity adversely affects the stability of the product.